Thousands of hazardous waste sites have been identified, many of which exhibit evidence of subsurface contamination of either or both of the vadose zone or the water table (aquifers). The bulk of the hazardous waste sites contain some level of toxic organic compound contamination, and many of the remediation techniques are directed toward this type of contamination. While not generally considered "toxic", nitrate contamination has also been identified as a significant groundwater contaminant in many aquifers, and remediation techniques are proposed to remediate nitrate contamination as well.
Organic chemicals enter subsurface regions either as aqueous solutes or as liquids that are immiscible with water (and have limited water solubility). Complicating the removal of such compounds is the fact that when organics enter as liquids, isolated bubbles of liquid form when the organic redistributes in the subsurface (unless otherwise designated, "subsurface" as used herein may apply to either or both of the vadose zone or an aquifer) and can act as a source of continuous resupply of the contaminant to the groundwater.
Methods of remediation that result in minimal environmental impact are obviously preferred. The use of microorganisms (either introduced or native) to degrade organic contaminants into CO2, harmless inorganics and biomass is currently viewed as one of the most promising techniques. However, a number of factors limit the effectiveness of this technique, namely: lack of adequate nutrients, lack of electron donors, lack of sufficient concentration of microorganisms to transform the target compound, lack of growth and energy sources, low bioavailability of the target compounds, and inability to deliver the microorganisms to the location of the contaminants. In many cases, the last of these factors can be the most difficult to overcome.
Organic compounds typically found as contaminants of groundwater are present as aqueous solutes or as immiscible liquids (in a separate phase). Because organic liquids commonly have some limited solubility in water, groundwater (aquifers) contaminated with organic liquids usually will contain aqueous organic solutes as well. Such solutes are known to sorb to the solid particles that constitute the matrix of the vadose zone or aquifer in a manner that retards their movement along with the flowing pore water. Therefore, the sorption process renders pump and treat remediation methods inefficient because extremely large volumes of water must be pumped in order to move the sorbing compounds to the extraction well.
Because of the relatively large mass of organic contaminant present in the liquid phase compared to its aqueous solubility, organic liquid phases act as extremely long-lived sources of aqueous contamination. Even if the organic liquid phase is found and removed, pockets, bubbles or ganglia of residual organic liquid remain trapped in the pore spaces and act as contaminant reservoirs for dissolution for extended periods of time.